Sena's overprotective and motherly childhood friend. Mamori's mother met Sena's mother in a cooking class, and Mamori and Sena have been friends ever since. Her mother is half-Japanese, half-American which makes Mamori 25% American and 75% Japanese. Mamori has been protecting Sena from bullies since primary school, to the point that Sena has grown up never having to stand up for himself. Mamori joined the American Football club to watch over Sena and prevent any bullying by the other members (particularly Hiruma). Despite her joining the American Football club just to protect Sena's claim, she eventually becomes an excellent manager who knows the rules inside and out and makes up strategies for the team (since she has trouble with computers, she does it all on paper—super-analog). She memorized all of american football rules in just 1 night and is able to answer Hiruma's question to the smallest detail. A lovely girl, Mamori excels in all her studies except art, which she is horrible at, and helped Sena with his high school entrance exam. She is also a member of the school's disciplinary committee. Though she is constantly butting heads with Hiruma, she eventually sees a softer side to the Devil Bats' quarterback.

Her favorite food is Italian food and creampuffs. She is kind of a "creampuff-maniac" since her personality drastically changes when the topic is about creampuffs (as seen in the anime and in manga chapter 206).